Don't use timestamp when generating ec2 signed url for alarm url
attributes of resource.
This will resolve the issue for new resources even if they are cached
in attributes or resource_data.
However, for existing stacks containing these resources the current
best option is to not cache these attributes and allow alarm resources
to be able to get a new never expiring urls in the next forced update.
Change-Id: If2ebc3deacb770294004ae023500367af603b59e
Task: 39985
Related-Bug: #1872737
d0e44ded0a